Water supplies for domestic drinking water, process water for chemical plants or other liquids are often contaminated with a variety of contaminants, such as toxic chemicals, algae, dissolved oil and various organic and inorganic particles of various sizes. These contaminants must be removed in a reliable, cost effective manner.
Many older water treatment plants use gravitational separation methods, typically in sedimentation systems or dual-media sand filtration systems that may not be acceptable under the newer water quality standards. In some cases, these systems can meet the standards through the use of properly mixed polymer chemical filter aids. The required expensive and complex polymer chemical mixing equipment requires constant attention, since the amount of the chemicals being added to raw water must be frequently readjusted to match the continually changing chemistry of the water being filtered. Slow sand filters require a considerable investment, but generally can be operate for longer periods without cleaning. Unfortunately, even with pre-treatment, both dual-media and slow sand filters fail to meet water quality standards for hours or several days after each backwash cleaning.
Ordinary chemical flocculation and sedimentation processes do not prevent toxic chemicals, pesticides and algae from passing through the ordinary filter bed. If algae spores are present when chlorine is added, toxic disinfection byproducts are formed, which is highly undesirable and a violation of the USEPA Safe Drinking Water Act. The inability of older municipal filtrations systems to remove algae is apparent in the lack of clarity found when a swimming pool is filled with "clean" tap water. Most pool contractors have to shock tap water with large doses of chlorine chemical pool oxidizer to achieve the desired clear pool water appearance.
Particulate material has also been removed from liquids by floatation, another gravitational method, in which bubbles of a gas, such as air or oxygen, are introduced into the lower levels of the liquid and float to the top, carrying fine particles with them. Various chemical additives, such as flocculation aids, typically alum polymers, are required with these systems. Flotation is a gravitational method because the rise of bubbles is due to the gravitational acceleration acting on the mass of the liquid in accordance with the basic force equals mass time acceleration relationship. A force balance relative to a pocket of gas phase within liquid (a bubble), where the mass of the bubble is its volume times its density, shows that the bubble must rise to find equilibrium, because the density of a gas is generally less than that of a liquid. Large flotation tanks are required to allow adequate time for air bubbles to reach the surface.
Failure to remove algae prior to filtration also leads to clogged filters, increases filter operation costs and wastes water required for frequent filter cleaning cycles. The use of flocculation promoting chemicals increases the volume of sludge to be dewatered and removed.
Thus, there is a continuing need for a separation system that will rapidly and efficiently remove particles and volatile gases from liquids while treating a liquid, will efficiently remove algae and volatile gases such as MTBE during pretreatment prior to filtration and will reduce overall treatment costs and conserves water through less frequent filter cleaning and a smaller sludge volume.